Drip irrigation run time and flow calculator

Before you rely on these numbers

• Pressure regulator. Thin-wall tape runs at 6–15 psi and 15 psi is the maximum for 8 mil. A 40–60 psi well or municipal line will split it, sometimes on the first run. A run time is useless advice for a system that ruptures.
• Filtration at 150–200 mesh. Emitters clog on particles and algae, and clogging destroys uniformity invisibly — the arithmetic still works while the crop starves. Budget 2–3 psi of head for a clean filter.
• Maximum run length. Non-pressure-compensating tape starves at the far end past its length-of-run limit, and slope makes it bite sooner. Get the figure from your tape manufacturer's chart for your flow rate, diameter and slope — published figures vary between vendors by more than a hundred feet at the same flow, so we do not guess at it here. Feeding a long bed from the centre halves the run.
• Measure one emitter. Time how long a single emitter takes to fill a measured container and compare it with the spec. It is the highest-value ten minutes you can spend on a drip system, and it catches both wrong assumptions and clogging.

How it works

1. Precipitation rate comes from flow and line spacing

   Rate (in/hr) = 0.963 × tape flow (GPM per 100 ft) ÷ line spacing (ft)

   Drip tape is sold by flow per hundred feet, and that flow spread over a narrower strip wets it faster. The constant is just unit conversion — a gallon over a square foot is 1.604 inches deep. The same answer comes out of the per-emitter form, which is a useful check on your own arithmetic.

2. Two lines in a bed do not each serve the whole bed

   Line spacing = bed width ÷ lines per bed

   Two lines in a 30 inch bed are 15 inches apart, so each one waters half the bed. Using the full bed width here is a factor-of-two error in the rate and therefore in every run time the tool produces. Note the consequence: doubling the lines halves the run time and doubles the flow, but the total water is unchanged.

3. Decide what the target depth applies to

   An inch of water over the wetted bed strip is a very different quantity from an inch over the whole field including paths. On typical market-garden spacing that choice is a 60% difference in gallons. Drip only wets the bed, so the wetted strip is usually right — but it has to be a decision, not an accident.

4. Flow decides how many zones you need

   System demand (GPM) = total tape length ÷ 100 × flow per 100 ft

   Add up every foot of tape running at once. If that exceeds what your well or meter can deliver, split it into equal-flow zones and irrigate them in sequence. Watch the total: zones multiply the time the valve has to be open, and if that runs past a sensible daily window the real problem is supply, not scheduling.

5. One application should not exceed what the soil holds

   Maximum per event = water capacity × root depth × allowable depletion

   Water beyond the root zone leaches nutrients past the crop. Split the weekly requirement into events sized to the soil — a sandy loam with a foot of roots takes roughly half an inch at a time, so an inch and a quarter a week is three irrigations, not one long one.

6. Efficiency, pressure and filtration decide whether any of it works

   Well-designed drip runs around 90% efficient, so plan to apply a bit more than the crop needs. Tape wants 6–15 psi and a 40–60 psi supply will split it, so a regulator is not optional. Nor is filtration at 150–200 mesh: clogged emitters ruin uniformity invisibly, and the numbers still look fine while the crop suffers.